Sadly you may be right... but only AFTER a complete crash fix for last gen.
They cannot simply leave last gen console players with an unplayable game.
they absolutely can
at least as of recent, the main problem for Console Edition is the Overlord update [with a secondary being the game is just having issues running on the older hardware]
assuming that Overlord is just the problem, and none of the other DLC in Pass 5 causes serious issues with the game [i.e., heavy crashing] they'd have to find a way (if it can even be done) to revert Consol Edition back to the Aquatics Patch, and refund everyone who bought Overlord [this in itself is a problem, because while it is easy for those who bought the DLC separately to get a refund, how would that even work for those who got the DLC via the Pass; if they refund the Pass, then everyone who bought it would lose access to Aquatics and Nemesis as well, given how refunds generally work...and that is assuming they can even get Microsoft and Sony on board with it; given how long it has been]
if the entire Pass is the issue, it pretty much is the same problem but bigger; attempting to revert the game back to the last patch of Expansion Pass 4 (assuming that can be done), getting Microsoft and Sony on board to refund Pass 5 [given how long it has been]
all in all, there's no guarantee they can fix the issue; and if they can, the amount of work that would have to be put in; by another company [as noted, Tantalus stopped working on Console Edition after Overlord] would just be effectively insurmountable
the easiest solution is ironically cut their losses, drop support for the older systems [PS4, Xbox One, and maybe even the Series S], and remake the game and base it around the hardware/software of the PS5 and Series X [not sure how they'd handle DLC, they could make it account locked so you'd just redownload it with the new edition once it comes out; they could just make packs 1-4 part of the new game install; or if they have to charge money for the passes again, have them severely marked down in cost for anyone who already owned the content on the older systems; and discount it for everyone else]
granted, this is all an opinion
do not take this as word of god [i.e., this is what is going to happen]
this is mainly just the ramblings of me trying to figure out some possibilities and explaining what would likely need to be done
